I was working on a character using mirror mode. I then created a separate object and joined it to the main mesh. However, after I tried moving this object, a few vertices from the end of the mesh would jump to the mirrored portion of the character:

I tried replicating this effect in other files, but it didn't always happen.

Is this a bug? Is anyone else experiencing it?

Note: This was in blender 2.77, with on additional plugins installed. The mesh had a subsurf modifier on as well, but the jumping occured even if I removed it.

$\begingroup$The possible reason is that joined object created separately was located a bit on the mirror plane of the Mirror modifier and then conflicted with Clipping option.$\endgroup$
– Mr ZakJun 22 '17 at 19:56